feat: rebuild GitDB runtime benchmarks and API examples

[3x-haust]

What changed

• Rebuilt GitDB around the first-party runtime/storage-engine contract instead of the old PostgreSQL facade positioning.
• Added API-style examples and removed the old examples/local-runtime surface.
• Added two encrypted examples: encrypted repository CRUD and encrypted reopen/wrong-key/plaintext-leak verification.
• Published benchmark evidence into the website and kept npm pack/publish dry-run setup intact.
• Fixed CI setup so GitHub Actions can run pnpm validation reliably.

Validation

• Local: corepack pnpm check, corepack pnpm test, corepack pnpm build, corepack pnpm example
• Benchmarks: GITDB_BENCH_ROWS=250 corepack pnpm benchmark:gate, GITDB_BENCH_ROWS=1000 corepack pnpm benchmark:gate, GITDB_BENCH_ROWS=250 corepack pnpm benchmark:compare
• Packaging: corepack pnpm pack:dry-run, corepack pnpm publish:dry-run
• Browser QA: static site on 127.0.0.1:4173, desktop/mobile screenshots, console errors = 0
• GitHub Actions: PR validate passed on run 27518921638